This invention relates to the measurement of the partial pressure of oxygen and more particularly to apparatus and method for continuously compensating for oxygen electrode drift.
Typical oxygen sensors consist of an anode and a cathode immersed in an electrolyte. The electrodes and electrolyte are contained within a membrane which blocks passage of the electrolyte but which allows molecular oxygen to pass through freely. One important oxygen sensor measures the oxygen which perfuses through the skin. In operation, such a transcutaneous sensor is placed against skin, for example, the wrist, which has been heated to cause hyperemia within the underlying capillaries. The increased blood flow elevates the capillary blood oxygen partial pressure to a level approaching that of arterial blood. Thus, the oxygen which perfuses from the capillaries through the heated skin gives an indication of the oxygen partial pressure of arterial blood.
Successive electrical voltage pulses are applied across the electrodes of such oxygen sensors thereby causing current to flow via three mechanisms. The first mechanism is the ion-electron transfer within the electrolyte. The second is the current flow associated with charging the so-called double layer at the electrode-electrolyte interface. This double layer may thus be thought of as acting as an electrical capacitor. The third mechanism, the one of interest, is current flow associated with the reduction of molecular oxygen. Thus, only part of total charge transferred to the cell during a voltage pulse is a function of the concentration of oxygen within the electrolyte. After the pulse, charge is returned from the cell, the amount of charge so returned being nearly independent of the oxygen partial pressure. The charge returned from the cell arises primarily from discharge of the double layer. Because the charge returned from the cell is nearly independent of oxygen concentration whereas the charge delivered to the cell is so dependent, the difference is proportional to the partial pressure of oxygen (po.sub.2) in solution. The use of this difference in inferring po.sub.2 is known as the net charge transfer technique. This difference between the charge delivered to and returned from the cell, however, is still subject to the very serious problem of oxygen electrode drift. Changes in the amount of charge returned from the cell from pulse to pulse thus indicate changes in the electrodes themselves which give rise to drift, since the charge so returned is nearly independent of the quantity to be measured--the partial pressure of oxygen in solution.
The drift or "aging" associated with oxygen electrodes has several origins. One cause of drift is the precipitation of insoluble salts on the electrode surfaces which reduce their effective area. Another cause is the attraction of large protein molecules to the cathode. Although considerable effort has been devoted to minimizing drift, its elimination has not been achieved. Heretofore, such electrode drift has necessitated frequent instrument calibration and recalibration, greatly reducing the utility of measuring the partial pressure of oxygen using an electrochemical cell.
It is an object of the present invention, therefore, to provide apparatus and method for continuously compensating for electrode drift in the net charge transport technique for determining oxygen partial pressure.